Technical Field
The present disclosure relates to power supply circuits in particular for portable devices, and in particular portable devices powered through a USB port (Universal Serial Bus) or by a battery rechargeable via such a port.
Description of the Related Art
A USB port is generally connected to a low dropout voltage regulator, enabling a regulated voltage generally comprised between 2.7 and 3.5V to be supplied. Such a circuit can receive varied voltages both low, of a few volts, and high, up to about twenty volts, with voltage peaks which can reach 30V. Furthermore, in a battery charger application, a voltage of a few volts may be present at the output of the regulation circuit, independently of the presence of the supply voltage to be regulated.
High voltage regulation circuits produced in CMOS technology have the disadvantage of implementing a specific power circuit comprising a high voltage MOS transistor (of drift or extended drain type), or cascode-mounting of transistors. Such a circuit is not suited to operate with a low consumption, in particular in a standby mode.
FIG. 1 represents an example of a conventional regulation circuit VRG1 linked to a USB port. The circuit VRG1 comprises a P-channel MOS transistor P11, comprising a source terminal receiving the voltage Vi to be regulated, supplied at input In of the circuit VRG1, a gate terminal connected to the output of an error amplifier CP11 and a drain terminal supplying a regulated voltage Vo at an output Out of the circuit VRG1. The error amplifier CP11 receives a reference voltage Vrf at a non-inverting input and, at an inverting input, a feedback voltage Vfb equal to a fraction of the output voltage Vo. The voltage Vfb is produced using a divider bridge comprising resistors R11, R12 mounted in series, the resistor R11 being connected between the non-inverting input and the output Out, the resistor R12 being connected in series with resistor R11 to the ground, and the voltage Vfb being supplied by the junction node between the resistors R11, R12. Generally, the output voltage is filtered by a filter capacitor C11 connected to ground. The transistor P11 is configured to tolerate high voltages at input and to supply a current, which may be high, to the charge supplied by the output voltage Vo. In operation, the transistor P11 is controlled to increase or reduce the current passing through it according to the difference Vrf−Vfb between the voltages Vrf and Vfb at input of the amplifier CP11. The assembly of the amplifier CP11, the transistor P11 and the resistor R11 thus forms a voltage regulation loop.
The circuit VRG1 is not provided to operate in low consumption or low power mode, i.e., with a low input voltage (for example lower than or equal to 5V) while consuming little current (for example lower than 100 μA). Furthermore, the circuits powered by such a power circuit must comprise a supplementary circuit using selector switches to adapt to the different high and low power supply configurations. In addition, the transistor P11, of drift or extended channel type, introduces into the circuit on its gate side, high stray capacitances imposing a low impedance on the output Out of the regulation circuit VRG1, which significantly influences the current consumption of the device integrating the circuit VRG1.
To solve this problem, proposals have been made to reduce bias currents in the regulation circuit by introducing several components having adjustable characteristics into it. The presence of these adjustable components in the regulation circuit renders the latter complex to control.
Proposals have also been made to introduce into the regulation circuit VRG1 a second regulation loop in parallel to the regulation loop formed by the amplifier CP11, the transistor P11 and the resistor R11. This solution requires a second high voltage transistor, and thus of a significant size, comparable to that of the transistor P11, to be capable of tolerating high input voltages. This solution also requires switch circuits to deactivate one or the other loop depending on the operating mode of the regulation circuit, as well as a relatively complex circuit for controlling these switch circuits, to avoid over voltages from forming when switching between the high and low power operating modes.
It is thus desirable to provide a voltage regulation circuit that is capable of operating at high and low power without any excessive consumption, particularly at low power. It may further be desirable to propose a voltage regulation circuit having a protection against short-circuits.